Previous devices, as disclosed in the Japanese Laid-open Patent Application No. 1-283099, have an encoder attached to the rotor of the stepping motor. However, a problem with these devices exists in that if the rotor vibrates without rotating, the device indicates that the stepping motor is out of step.
In these prior-art devices, however, a mechanical connector forming a closed loop must be present between the stepping motor and the encoder to feed detection signals from the encoder back to the stepping motor. As a result, the stepping motor is unfavorably large in size.
In an electric power steering system using the stepping motor, for example, the large size of the stepping motor restricts the location of the power steering component parts, and increases costs as a result.